System and Method for Quantifying Fragile X Mental Retardiation 1 Protein in Tissue and Blood Samples

ABSTRACT

A system and method for the detection and quantification of fragile X mental retardation protein (FMRP) in human tissue and blood samples. The system includes several high avidity monoclonal antibodies that may be provided on Xmap microspheres to capture FMRP from a tissue or blood specimen. The resulting complex is reacted with a polyclonal anti-FMRP rabbit antibody and then mixed with an anti-rabbit IgG antibody conjugated to phycoerythrin. Fluorescence emitted from the resulting complex is a function of the amount of FMRP present in the specimen.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority to U.S. Provisional ApplicationNo. 61/495,679, filed on Jun. 10, 2011.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to fragile X mental retardation 1 proteindetection and, more specifically, to a system and method for quantifyingthe protein in tissue samples in a capture immunoassay using anti-FMRPantibodies.

2. Description of the Related Art

Fragile X syndrome (FXS) is a heritable condition characterized bycognitive and behavioral abnormalities and is the most common singlegene cause of autism. The syndrome results from an expanded triplet CGGrepeat that generates a fragile X site on the X chromosome that resultsin the absence or reduced expression of the FMR1 gene product FMRP.Almost all individuals with the syndrome carry FMR1 forms (alleles) thatdo not express the protein because the mutated alleles harbor longstretches of hypermethylated CGG repeats, e.g., the full mutation (FM)has more than 200 CGG repeats, that abolish or compromise FMRItranscription and/or translation. Alleles containing shorter repeats,e.g., permutations of 55 to 200 CGG repeats, may express a reducedamount of FMRP.

Most individuals carrying the permutation (PM) are not cognitivelyaffected. However, PM alleles have been reported to play a role inautism spectrum disorders, premature ovarian failure and fragileX-associated tremor-ataxia syndrome. Early diagnosis of the syndrome isextremely important for child supportive care, early intervention andfor family planning.

The laboratory diagnosis of Fragile X syndrome is currently performed byDNA testing (Southern blot and PCR methods) using blood or othertissues. The tests often require several days (7-10 days), however, andcan be performed only by a limited number of specialized laboratories.These methods are directed at determining the length of the CGG repeatsin the FMR1 alleles and use the CGG repeat size of the allele to inferor predict whether the proband cells produce abnormal levels of FMRP.

The development of an immunoassay for the direct quantification of FMRPhas been hampered by the lack of mouse monoclonal antibodies (mAbs)having the affinity required to capture efficiently the human proteinwhile showing no cross reactivity with the Fragile X related proteins,FXR1P and FXR2P. Various attempts have been made to test directly forthe presence of FMRP in blood and other tissues using mAb IC3 thatrecognizes an epitope localized in the N-terminal of FMRP. The problemwith these tests, however, is that the 1C3 mAb cross-reacts with the FXrelated protein FXRI and does not have a strong binding affinity toFMRP.

For example, one test uses a Western blot analysis to study FMRPexpression in human lymphocytes from FM patients, PM and normalindividuals. This test uses an anti-FMRP mAbla from Chemicon that crossreacts with a 70 kd protein (FXR1) in FM male samples.Immunocytochemical staining of lymphocytes or hair root cells using ananti-FMRP mAb (1C3) has also been used to detect cells expressing FMRP.Cells from male FM patients are not stained or show a small percentageof stained cells. The test does not measure the quantity of FMRP, butinstead determines the fraction of cells that express the protein.

Finally, a luminometer-based sandwich ELISA for FMRP that allowsquantification of FMRP in lymphocytes within 3 to 4 days has beendescribed in the art. The assay uses a chicken polyclonal Ab to captureFMRP and a commercially available mAb,1C3 (Chemicon, Millipore) fordetection. Because of cross-reacting, this test produces a highbackground that does not allow signal detection when used with commonblocking agents as milk and BSA. While the background may be reduced orsuppressed using hydrolyzed casein as blocking agent, the blockingstep—usually performed before incubation with the antigen—must followthe incubation with the antigen (lymphocyte extract) in order to producea detectable signal and the ability of the chicken polyclonal Ab tocapture FMRP from the lymphocyte extracts is reduced or abrogated by thepresence of the blocking agent. In addition, this ELISA is cumbersome,time consuming (around 3 to 4 days), and requires several longincubation times, such as 24-48 hours for binding of the chicken Ab tothe plate, overnight incubation for binding of antigen, about 2 hoursfor blocking, 8-10 hours for binding with the detecting mAb, and thenabout 12 hour for binding with horseradish peroxydase-conjugated donkeyanti-mouse IgG. Accordingly, there remains a need for an immunoassaythat is relatively fast and does not require a specialized laboratory.

BRIEF SUMMARY OF THE INVENTION

The present invention comprises a series of specific mAbs (6B8, 1B12,5C2, 2D10, 10H12, etc.) that have high affinity to human FMRP and thatshow no cross-reactivity to FXR1P or FXR2P. The present invention alsoincludes a polyclonal antibody (R477) generated by immunizing a rabbitwith a peptide (DDHSRTDNRPRNPREAK) (SEQ. ID NO. 1) corresponding to acarboxyl domain (starting at aa. residue 554) of human FMRP. R477 Abbinds with high specificity and avidity to FMRP.

The present invention further comprises a Xmap microsphere (Luminex)based, enzyme-linked immunosorbent assay (ELISA) that allows thedetection and quantification of FMRP in blood samples and other tissues.First, the Xmap microspheres are coated with any of the above mentionedhigh avidity mAbs to capture FMRP from the specimen. Next, themicrosphere-mAb-FMRP complex is reacted with anti-FMRP R477 Ab. Finally,anti-rabbit IgG Ab conjugated to phycoerythrin is added. Thefluorescence emitted from the result of this process is a function ofthe amount of FMRP present in the specimen and may be detected using theLuminex-200 System.

The assay may be used to detect FMRP in human lymphocytes, platelets,dried blood spots, cultured chorionic villi cells, lymphoblastoid celllines, mouse or human brain extracts and other tissues. As expected bythose of skill in the art, the assay does not detect FMRP in lymphocytesisolated from full mutation (FM) male Fragile X patients, inlymphoblastoid cell lines derived from male FM FX individuals, and inbrain extracts from the Fmr1 KO mouse. Low levels of FMRP are detectedin specimens derived from male FM size mosaic and methylation mosaicpatients.

Micropheres are available in 100 distinctly colored sets each exhibitinga unique signature that is recognized by the Luminex System. Using twoor three different microsphere sets, where each set is coupled with adifferent specific anti-FMRP mAb, the present invention provides amultiplex capture sandwich immunoassay for the detection in the samewell of FMRP by more than one antibody. The FMRP signal is compared tothat of bona-fide lysates from normal individuals (or mice) and isquantified using as standards recombinant fusion proteins carrying onlythe capture and detection domains of FMRP. The assays may be performedin 96-well-plates and can be completed in 24 hours.

The present invention may be used to detect and quantify FMRP from driedblood spots (DBS). In this method, drops of blood are spotted onto acollection card (Whatman, WB10001.4) and air-dried for a few hours. Thematrix in the card lyses cells, denatures proteins and inhibits thegrowth of microorganisms. While blood samples (6 to 8 ml) must be storedin a refrigerator and processed for isolation of lymphocytes orplatelets as soon as possible (hours from acquisition), DBS cardspreparation requires collection of small blood samples (about 20 μl perspot) that are drawn by lancet from the finger, heel, or toe of aneonate. The cards can be stored at room temperature in lowgas-permeability plastic bags and sent by regular mail to a laboratoryfor testing. DBS are used routinely for neonates screening of metabolicdiseases, serum protein levels, intracellular enzymes, viruses, andgenetic disorders. Moreover, measurement of FMRP in DBS can be performedalong with other metabolic markers in a multiplex capture Lumineximmunoassay for simultaneous measurement of a wide range of proteinsfound in blood such as the conventional inflammatory markers orneurotrophins, regularly performed on neonatal dried blood spots byimmunoassay with the xMap technology. Our assay can easily beincorporated into a routine protein marker screening of neonatal DBS.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S)

The present invention will be more fully understood and appreciated byreading the following Detailed Description in conjunction with theaccompanying drawings, in which:

FIG. 1 is a schematic of the system and method for the detection of FMRPin a tissue sample according to the present invention;

FIG. 2 is a graph of the median fluorescence intensity verses theconcentration of total protein for mAb 5C2 using mouse brain tissuesamples;

FIG. 3 is a graph of the median fluorescence intensity versesconcentration of total protein for mAb 5C2 using a human lymphoblastoidcell line derived from a normal individual;

FIG. 4 is a chart of median fluorescence intensity verses total amountof protein extract from human lymphoblastoid cells (normal control andFM FX male patient) using mAb 5C2;

FIG. 5 is a chart of median fluorescence intensity detection of FMRP inseveral control (N1-N8) and target lymphoblastoid cell lines using mAb5C2.;

FIG. 6 is a histogram of FMRP detection in DBS where the MSI values wereproduced from DBS from normal, premutation and FX individuals andcapture was done using mAb 6B8 coupled microspheres. More specifically,specimens 140wt, 139.1wt are from normal male individuals and were usedas positive controls; Balb/cJ mouse brain homogenate was used asnegative control since mouse FMRP does not bind to mAb 6B8; negativecontrol was done without FMRP; specimens 142MfxFM, 154MfxFM, 139.2MfxFmare from male full mutation FX individuals; specimen 150MfmMos is from amale mosaic full mutation individual; specimens 124Fpm and 155BFpm arefrom female premutation individuals; and the other specimens are fromindividuals whose FX status had not been determined;

FIGS. 7A and 7B are graphs of median fluorescence intensity versesconcentration of total protein from lymphocytes of a normal individualwhere both experiments were done using three sets of X-map microsphereseach coupled to one mAb, i.e., 5C2, 6B8, and 2D10. FIG. 7A depicts anexperiment performed using capture mAb in a separate well (simplex) andFIG. 7B depicts an experiment performed using all three mAbs in the samewell (multiplex).

FIG. 8 is a chart of median fluorescence intensity for the detection ofFMRP using mAb 6B8 coupled x-Map microspheres to capture where specimens72G2 and 72D1are from normal individuals and 72Fxmale is from a male FMFx patient, and capture was done using mAb 6B8;

FIG. 9 is a chart of median fluorescence intensity showing the detectionof FMRP in lymphocyte extracts from nine normal individuals wherecapture was done using mAb 6B8;

FIG. 10 is a chart of median fluorescence intensity showing thedetection of FMRP in lymphocyte extracts f from six normal individualswhere capture was done using mAb 6B8;

FIG. 11 is a chart of FMRP values relative to an internal standard forseveral specimens using mAb 6B8;

FIG. 12 is a chart of fluorescence detection of FMRP in lymphocyteextracts from normal individuals (95-1 and 77-8) and from threepermutation women (pre99F, pre100Fa and pre100Fe where capture was donewith mircospheres coupled to mAb 6B8;

FIG. 13 is a chart of FMRP values relative to an internal standard (abona-fide lymphocyte extract) for specimens described in FIG. 12.

FIG. 14A-C is a series of Western blots showing the capture assay pairwith lymphocyte extracts from a FXS patient using (A) mAb68B, (B) R477,and (C) mAb1C3 with overexposed films in the middle section and sampleloading shown in the bottom section with an anti-GAPDH antibody.

FIG. 15 is a schematic of the mAb6B8 and R477 epitope domains;

FIG. 16 is a graph of the dose response curve of fluorescence (MFI) as afunction of GST-SR7 concentration;

FIG. 17 is a series of graphs of the levels of FMRP in DBS with variableexpression of FMRP in normal individuals (black, N=134) and in normalplus permutation individuals (grey, N=195);

FIG. 18 is plot of FMRP levels in normal individuals as a function ofage;

FIG. 19 is a series of charts comparing FMRP expression in males, where(A) is full mutation (N=17) and normal (N=85), and (B) is mosaic (N=7)and non-mosaic (N=10) full mutation groups;

FIG. 20 is a Southern blot analysis of genomic DNA isolated from anormal male (lane 1), a FM male (lane 2) and two mosaic FM males (lanes3 and 4);

FIG. 21 is a chart of a comparison of FMRP level in females with fullmutation alleles to females with normal or permutation alleles (N=88)using the Mann-Whitney test, where (A) is a plot analysis showingsignificantly lower levels of FMRP in full mutation (p=0.03) and (B) isa Receiver-Operating Characteristic (ROC) curve showing that at a cutoffof 13.7 pM, the assay for full mutation females gives a sensitivity of20% and a specificity of 93%.

DETAILED DESCRIPTION OF THE INVENTION

Referring to the Figures, wherein like numerals refer to like partsthroughout, there is seen in FIG. 1 a pathway for the detection andmeasurement of FRMP according to the present invention. Moreparticularly, anti-FMRP mAbs (preferably bound to Xmap microspherebeads) are mixed with a target tissue sample extract containing FMRP toform an antibody FMRP complex bound to the beads. Rabbit anti-FMRPantibody is then attached to the complex. An anti-rabbit IgG Abconjugated to phycoerthrin is then added so that the amount ofphycoerthrin fluorescence may be measured to determine the quantity ofFMRP bound to the beads.

As described above, the present invention includes a number of newanti-FMRP monoclonal antibodies that exhibit a high affinity for thecapture of human FMRP without exhibiting cross-reactivity to relatedproteins FXRP1 or FXRP2. The mAbs according to the present inventionwere produced in the IBR Monoclonal Antibodies Facility usingrecombinant mouse FMRP and human FMRP. These proteins were expressed inSf9 insect cells infected with baculovirus engineered to carry eitherthe mouse or the human FMR1 gene. The recombinant proteins wereexpressed, purified in large scale, and used for immunizing mice and forhybridoma screening. Several rabbits were immunized with a FMRP peptide(17-aa long position 554 to 570), i.e., (DDHSRTDNRPRNPREAK) (SEQ. ID NO.1). A rabbit anti-FMRP R477 polyclonal antibody produces a particularlystrong response with high affinity and avidity to the protein. Thepurified R477 Ab reacts strongly to the immunogen peptide and FMRP froma variety of sources:including mouse brain extracts, human lymphocyteextracts, platelets, lymphoblastoid cell lines, lymphocyte extracts anddried blood spots. This antibody serves as the detection reagent in theprotein-based Luminex assay.

The epitopes of mAbs 1B12 and 6B8 have been determined (see Table 1below). The other mAbs have been found to bind to a domain of FMRP(Table 1).

TABLE 1 FMRP Domain mAb (aa) Epitope 1F1 442-540 NA 1E3 320-354 NA 1B12329-375 GPNA/SP/SEE  (SEQ. ID NO. 2) 2G5  76-132 NA 3E7 442-540 NA 5C2320-375 NA 2D10 396-431 NA 3H8 320-375 NA 10H12 442-540 NA 6B8 320-375SRVGPN (SEQ. ID NO. 3) 7F9 442-540 NA

Table 2 below illustrates the phycoerythrin fluorescence measured as afunction of the total amount of protein for two of the antibodies 5C2and 2D10 of the present invention, and with an anti-PrP mAb, 3F4. Thelatter mAb, as expected by people of skill in the art, did not reactwith FMRP and was used as negative control.

TABLE 2 Antigen μg Mouse Brain H Lymphobl control H Lymphobl FX maleCapture Beads = 5C2 80 1879 2106 14 629 601 13.5 27 18.5 16 40 1475 151416 477 460 17 20.5 20 16 20 835 899 18 313 307 18 20 19 16 10 459 449 18184 181 17 22 22 21 5 226 216 22 80 88 18 24 23 22.5 2.5 116 113 22 5658.5 22 25 23.5 23.5 1.25 74 61 12 38 37.5 22 22 26 22 0 34 32 31 28 3326 27 24 26 Capture Beads = 3F4, anti PrP 80 51 49 4 6 6 3 5 4 3 40 3639.5 4 5 6 4 4 4 4 20 17.5 18 4 5 6 4 4 5 4 10 11 10 4 5 4 4 5 5 4 5 7 83 5 5 3 5 6 4 2.5 7 6 0 6 6 4 5 5 5 1.25 7 5 0 5 5 5 5 4.5 4 0 6 6 4 6 65 6 5 4 Capture Beads = 2D10 80 447 528 26 275 317 28 35 32 28 40 289309 28 181 182 28 35 34 32 20 138 139 31 94.5 111 32 33 32.5 28 10 72 6831 64 66 32 35 34 30 5 47 47 29.5 47.5 45 32.5 33 31.5 29 2.5 40 41 25.541 41 34 31 30.5 28 1.25 41 27 0 36 35 32 29 31 28 0 38 39 39 35 38 3134 31 30

As seen in the above Table 2 and in FIGS. 2-8, mAb 2D10 reacts to bothhuman and mouse FMRP. mAb 6B8 reacts strongly to human, but not mouseFMRP. Both of these antibodies were derived from mice immunized withhuman recombinant FMRP. Additional antibodies (see Table 1) include mAb5C2 and 1B12 derived from mice immunized with mouse recombinant FMRP arereactive to both mouse and human FMRP.

There is seen in FIGS. 4 and 5 an assay displaying the MFI verses totalprotein amount of lymphoblastoid cells extract from a normal individualand from a FM FX patient. Whereas the MFI obtained from the normalspecimen was dependent on the amount of total protein, the MFI obtainedwith the FX specimen did not show any variation.

There is in FIG. 6 an assay done using Dried Blood Spots and mAb 6B8Xx-Map microspheres. Specimens from normal male individuals were used aspositive controls and produced MFI values around 750. A mouse specimenwas used as negative control since the capturing 6B8 microspheres do notbind to mouse FMRP. Specimens from full mutation FX males produced MFIvalues of 10 to 30. A specimen from a mosaic FM FX patient showed a MFIof 150. Premutation samples showed either normal MFI values or slightlower MFI. The results clearly indicate to those that know the art thatthe DBS assay can be used to screen for male FM FX and male mosaic FMindividuals.

There is shown in FIGS. 7A and 7B assays performed with three diverseX-Map microspheres coupled either with mAb 5C2, 6B8, or 2D10. Twoexperiments were performed using a normal lymphoblastoid cell lineextract. In one experiment, each set of coupled microspheres were placedin separate wells (Simplex assay), in the other all three sets were usedin the same wells (Multiplex assay). In both experiments, microspherescoupled with mAb 6B8 produced higher MFI than the other two sets. TheMultiplex experiments shows that when used in the same well 6B8microspheres still maintain high MFI while the other two sets producelower MFI with respect to those obtained in the Simplex assay. To thoseof skill in the art, the data indicate that mAb 6B8 has higher affinityto human FMRP than the other two mAbs. Table 3 below describes thespecimens used in FIG. 5.

TABLE 3 Sample Genotype Sex CGG MFI Cell line N1 Control male normal1060 GM4736 N2 Control female normal 851 GM7000 N3 Control NA normal 691GM7113 N4 Control male normal 1041 GM6994 N5 Control female normal 723GM7010 N6 Control female normal 1047 GM7036 N7 Control male normal 691GM7352 N8 Control female normal 919 GM7436 82m mosaic male 82, >200 97C4079 65m premutation male 65 901 C6711 FM full mut male >200  80 C2159FF full mut female 30, >200 35 C8027 90m premutation male 90 249 C390770m Permutation male 70 1236 C5557 80m Permutation male 80 932 C2274 70mPermutation male 70 1450 C3137 MOSm mosaic mut male >200  331 C10148

There is seen in FIG. 8, an assay displaying the median fluorescenceintensity (MFI) (the median Fluorescene value detected by the systemafter reading 100 microspheres) for two normal control and one Fragile Xmale lymphocyte extract. Note the nearly zero MFI for the Fragile Xlymphocyte extract compared to the 100 fold higher values for the twocontrol lymphocyte extracts. This 100 fold difference is highlyreproducible relative to male full mutation FX individuals.

There is seen in FIGS. 8 -10 the detection of FMRP in lymphocytesderived from the normal population (14 samples) where MFIs varied from550 to 800. FIG. 11 depicts these values are compared to the internallymphocyte standard 77-5. Despite the use of identical proteinconcentrations for these assays, there is a wide variation in the MFIlevels. The cause of these variations are not known.

There is seen in FIGS. 12 and 13 the MF1s for lymphocyte extracts fromtwo controls and three premutation females. Note the intermediate MFIsfor the lymphocyte extracts from the premutation females compared to theextracts from the control subjects. These figures reflect the MFIsrelative to the internal lymhocyte standard. While the MFIs for allthree premutation females were lower than the two control MFIs, thevariation in control MFI values does not allow a statistical comparison.

EXAMPLE 1

The method for detecting and measuring FMRP according to the presentinvention may be used in connection with human lymphocytes as follows.First, mononuclear cells are isolated from blood using BD Vacutainer CPT(Becton Drive, Franklin Lakes, REF 362760) according to the BD protocol.Cells are lysed in M-Per (Invitrogen) buffer supplemented with 150 mMNaCl, chymostatin (1O μg/ml), antipain (10 μg/ml, and 1/200 dilution ofProtease inhibitor cocktail set III (Calbiochem), and sonicated using aBranson Digital Sonifier 250 (Denbury, Conn.). Cell debris are removedby centrifugation at 16,000×g at 4° C. for 15 min and the proteinconcentration is determined (Micro BCA Assay kit; Thermo Scientific,Rockford, Ill.). Samples are diluted in assay buffer (PBS, 1% BSA, 0.05%Tween 20, 0.05% Na Azide).

Luminex-based ELISA protocols are well known to those of skill in theart, and will not be described here in detail. In brief, microspheres(about 5,000) that are coupled with the chosen anti-FMRP mAb are mixedin a well of a Multiscreen Filter Plate (MSBVN1210—Millipore, Billerica,Mass.) with the lysates (100 μl total volume) and incubated on a plateshaker at room temperature in the dark for 4 hours. A serial dilution ofa lymphocyte extract (in duplicate wells, starting at 5 μg total proteinper well) from a normal individual and/or a recombinant fusion proteinare/is used as standards to quantify FMRP. The supernatant is aspiratedusing a vacuum manifold, the microspheres are washed 3 times,re-suspended in 100 μl of assay buffer containing 2 μg/ml of R477 Ab,and incubated on a plate shaker overnight. The liquid is aspirated, themicrospheres washed and incubated with 100 μl of anti-rabbit IgGconjugated to phycoerythrin (2 μg/ml in assay buffer; InvitrogenP2771MP) for 2 hours. After aspirating the liquid, microspheres arere-suspended in 100 μl of assay buffer and analyzed on the Luminex 200.

EXAMPLE 2

The present invention may also be used in connection with lymphoblastoidcell lines as follows. First, pellets of cultured cells (1×107) arehomogenized in a dounce homogenizer (20 strokes with the loose pestle)in 1 ml of ice cold buffer (10 mM HEPES H 7.4; 200 mM NaCl, 0.5%TritonX-100, 30 mM EDTA, Protease inhibitor) available from Roche.Lysates are transferred into tubes, centrifuged at 6,800×g at 4° C.Supernatant is transferred into fresh tubes, and protein concentrationdetermined as described above. The Luminex capture ELISA is performed asdescribed in above with respect to human lymphocytes.

EXAMPLE 3

The present invention may additionally be used in connection with driedblood spots (DBS). The preparation and use of DBS is well known to thoseof skill in the art, and will not be described here in detail. In brief,blood samples are spotted onto ID blood staining cards (Whatman,WB100014) and let dry overnight at room temperature. Cards are thenwrapped in aluminum foil and stored in sealed plastic bags at roomtemperature. Disks (7-mm-diameter) are punched from dried blood spotsand transferred into a 2-mi screw-cap tube with wide bottom. M-Perbuffer (150 μl) supplemented with 150 mM NaCl, chymostatin (10 μg/ml),antipain (10 μg/ml, and 1/200 dilution of Protease inhibitor cocktailset III (Calbiochem) is added and tubes incubated at room temperature ona Belly Dancer at maximum speed for 3 hours. Tubes are centrifuged for20 sec and supernatant transferred into a fresh tube. Debris is removedby a brief centrifugation step and supernatant (20 μl to 40 μl) dilutedin assay buffer. FMRP is detected using the capture ELISA describedabove.

EXAMPLE 4

The present invention may further be used in connection with platelets.Isolation procedures of human platelets are well known to those of skillin the art and will not be described in detail herein. In brief, bloodsamples (8 ml) are collected in a vacutainer BD tube (yellow cap, 39 mMcitric acid, 75 mM sodium citrate, 135 mM glucose, pH 7.4), andcentrifuged at low speed (190×g) for 15 min at room temperature.Platelet-rich plasma is transferred to fresh tubes and centrifuged at2500×g for 5 min at room temperature. The platelet pellets are lysed inM-Per (Invitrogen) buffer supplemented with 150 mM NaCl, chymostatin (10μg/ml), antipain (10 μg/ml, and 1/200 dilution of Protease inhibitorcocktail set III (Calbiochem), and sonicated using a Branson DigitalSonifier 250. Debris is removed by centrifugation at 16,000×g at 4° C.for 15 min and protein concentration determined as described above. TheLuminex capture ELISA is performed as described above.

EXAMPLE 5

The present assay may be used in connection with mouse brain extracts.Brains are washed in cold PBS and homogenized using in a douncehomogenizer (10 strokes with the loose pestle) in 2 ml/brain of ice coldbuffer (10 mM HEPES pH 7.4; 200 mM NaCl, 0.5% TritonX-100, 30 mM EDTA,protease inhibitor (Roche, complete). Debris is removed bycentrifugation at 6,800×g at 4° C. Supernatants are transferred to freshtubes and NaCl is added to a final concentration of 400 mM. Supernatantsare clarified by centrifugation at 50,000×g in a Beckman TLA 100.4 rotorfor 30 min at 4° C. After protein determination, samples are assayed forFMRP using the Luminex capture ELISA as described above.

The capture ELISA method of the present invention allows for detectionand quantification of a low abundant intracellular non-enzymatic proteinin DBS. Thus, the invention can be applied in the detection andquantification of other relevant intracellular proteins by developingspecific-ad hoc-capture immunoassays. Compared to conventional tests,the present invention has a very high signal to background ratio, workswith BSA as blocking buffer which is used in all steps (before, duringand after the capture step), has few and shorter incubation times (about4 hours with the capture mAb, overnight with the detecting rabbit Ab,and the 2 hours with the anti-rabbit IgG Ab conjugated tophycoerythrin).Thus this assay can be performed in 24 hours and usesreplenishable, specific, high-affinity anti-FMRP mAbs. Furthermore, thepresent invention may be used with the Luminex platform that allows formultiplex formats and several anti-FMRP mAbs can be used to detect FMRPin the same sample. Multiplex can be also performed with microspheresets coupled with mAbs against other antigens. Multiplex assays can alsobe used as negative controls, or to detect diverse antigens for samplenormalization.

The present invention is particularly valuable as it allows fordetection and quantification of FMRP in the minute amount of proteinsextracted from dried blood. The ELISA can identify male FM FX, malemosaic FM FX, from normal and from PM individuals (see FIG. 6). In asmuch as newborn DBS acquisition and screening for metabolicabnormalities is performed routinely in most of the U.S., and in a largenumber of countries in Europe, Americas and Australia, the presentinvention offers for the first time a valuable, simple, economical andaccurate method to perform worldwide newborn screening for FXS in thegeneral population.

Furthermore, the present invention provides a method that, for the firsttime, can detect and quantify low abundance intracellular non-enzymaticproteins in DBS by capture immunoassays. Therefore, the presentinvention can be applied in the diagnostic detection and quantificationof other relevant intracellular proteins present in otherneurodegenerative diseases. like the Batten Cln3 protein, which is notpresent in 85 percent of Batten Disease patients. The present inventioncan also be used to quantify FMRP in human chorionic villi and otherorgans and tissues, as well as to study FMRP expression at differentstages of development.

EXAMPLE 6

The recombinant fusion protein for FMRP quantification was developed aspart of this invention A glutathione S-transferase (GST) fusion proteincarrying two short domains of FMRP corresponding to the epitopes ofmAb6B8 and R477 was constructed with a double stranded syntheticoligomer encoding a nine amino acid sequence of FMRP (aa 344 to 352)that includes the epitope recognized by mAb6B8. The double strandedproduct, which was flanked by a 5′ BamHI overhang and a 3′ EcoRIoverhang was ligated into vector pGEX-4T (GE Healthcare Biosciences,Piscataway, N.J.).

This plasmid, which expressed a peptide recognized by 6B8, was modifiedto include the R477 epitope as follows. The FMR1 cDNA region thatencodes the R477 epitope was amplified with forward and reverse primersCGGAATTCCGTGGAGGAGGCTTCAA (SEQ. ID NO. 4), CCCTCGAGCAGCCGACTACCTTCCACTG(SEQ. ID NO. 5) and ligated the amplimer downstream of the 6B8 epitopeto generate the plasmid pGEX-hFMR1-S. Clones were screened by westernblot analysis for expression of a fusion protein, GST-SR7, that reactedwith both antibodies. The fusion protein was expressed in E. coli strainBL21 by IPTG induction and purified by glutathione-Superflow resin(Clontech) according to the manufacturer's directions. After elutionwith 10 mM glutathione in 0.1M Tris-HCl pH 8.0 solution, GST-SR7 wasdialyzed against 25 mM Tris-HCl pH 7.4, 150 mM NaCl buffer, concentratedin an Amicon Ultra-15 10K (Millipore), aliquoted, lyophilized and storedat −70° C.

The FMRP concentration was determined with serial dilutions of therecombinant fusion protein GST-SR7 (FIG. 16) to generate a standardcurve for each plate using the MasterPlex QT v2.5 quantitative analysissoftware for protein assay (MiraiBio, Hitachi Solutions America Ltd,South San Francisco, Calif.). Median fluorescence intensity (MFI) wasplotted against recombinant FMRP concentrations using a sigmoidal fiveparameter logistic model.

A Western blot analysis (FIG. 14) with protein samples (15 μg) analyzedon precast 4%-15% polyacrylamide Criterion Tris-HCl gels (BioRad,Hercules, Calif.) at 200 mV for 1 hr according to the manufacturer'sdirections. Separated proteins were transferred onto PVDF membranes(0.22 um, BioRad, Hercules, Calif.) in transfer buffer (25 mM Tris, 192mM glycine, pH 8.3) using a semidry electroblotter (OWL HEP-1, ThermoScientific, Waltham, Mass.) for 1 hr at 10 V. Membranes were incubatedin 5% nonfat dry milk in 0.01 M Tris pH 7.5; 0.137 M NaCl; 0.05% Tween20 (blocking buffer) and then incubated with either anti-FMRP antibodies(mAb6B8, R477, or mAb 1C3 from EMD Millipore, Billerica, Mass.), or arabbit anti-glyceraldehyde 3-phosphate dehydrogenase (0.2 μg/ml,sc-25778, Santa Cruz Biotechnology, Inc. Santa Cruz, Calif.). Afterwashing, membranes were incubated for 1 hr with the conspecific alkalinephosphatase-conjugated secondary antibodies (Sigma-Aldrich Corp., St.Louis, Mo.). Proteins were detected with CDP-Star (NEB, Ipswich, Mass.)according to the manufacturer's directions.

The fragile X analysis of DNA isolated from blood samples (FIG. 22) wasperformed by PCR and Southern blot with the AmplideX® FMR1 PCR (RUO)reagents and capillary electrophoresis according to the manufacturer'sdirections (Asuragen, Austin, Tex. 78744 USA). DBS DNA was isolated withthe DNeasy kit (Qiagen, Valencia, Calif.) according to themanufacturer's directions for QIAamp DNA Mini Kit, concentrated byprecipitation with ethanol and analyzed with AmplideX® FMR1 PCR (RUO)reagents as above.

DBS from blood received more than 3 days after collection were excludedfrom the analysis. The single newborn blood sample DBS was alsoexcluded. Data were analyzed with either SPSS (Chicago, Ill.) orSigmaPlot (Systat Software Inc., San Jose, Calif.) software.

As described above, initial experiments suggested that mouse monoclonalantibody (mAb) 6B8 and rabbit polyclonal R477 that had been developed inaccordance with the present invention were the best candidate pair. Thespecificity of mAb6B8 and R477 for FMRP seen in FIG. 14 wascharacterized by western blot analysis of extracts from normal (male andfemale), premutation (female), and full mutation (male) lymphocytes.Three major FMRP bands (68 to 80 kDa) were recognized by mAb6B8 innormal and premutation samples while no bands were detected by thisantibody in the full mutation FXS male sample. This indicated that thisantibody has little if any cross reactivity with the closely relatedproteins FXR1P and FXR2P or other unidentified proteins. Three bands(66-80 kDa) were detected with R477 in all extracts except male fullmutation FXS. As seen in FIG. 14, this antibody also detected a faintband (65 kDa) in all lanes (normal, premutation and male FXS), that wasvisible on long exposure, indicating that R477 has a weak crossreactivity to an unspecified protein. Since the western blot dataindicated that these antibodies both recognize FMRP without having anyother targets in common, the combination of mAb6B8 and R477 as captureand detection reagents, respectively, should allow highly specificdetection of FMRP in a capture immunoassay.

As illustrated in FIG. 4, the levels of FMRP detected (MFI) in the innormal cells were strictly proportional to the amount of sample.Background fluorescence values were detected in all wells containingmale full mutation extracts, independently of the amount of sampletested. In normal individuals, N1 to N8 in FIG. 5, FMRP levels vary from691 MFI and 1060 MFI. Similar values were measured in 4 premutationmales (901 MFI to 1450 MFI). While background fluorescence was detectedin a FM cell line (FM, 35 MFI), low amount of FMRP was found in a celllines derived from two males full mutation mosaic (FM mos 1, 97 MFI, mos2, 331 MF).

For FMRP quantification, as well as to control for technical variationsin capture and detection, a reference protein was constructed for theLuminex immunoassay. Referring to FIG. 15, a series of GST-fusionproteins were used that carry discrete regions of FMRP to localize themAb6B8 and R477 epitopes (LaFauci et al.; manuscript in preparation) anda GST fusion protein was constructed, GST-SR7, that harbored both ofthem. Purified GST-SR7 at concentrations of 0.5 to 300 pM resulted in alinear response in the Luminex assay (see FIG. 16). Repeated assays (55)of GST-SR7 at different concentrations were highly correlated (r=0.996)and long term (4 month) storage of the protein standard at −70° C. didnot affect its performance which indicated that this standard was stableand not prone to aggregation.

Referring to FIG. 16, a standard curve calculated from dilutions of aknown amount of GST-SR7 was used to measure the amount of FMRP presentin DBS samples from 215 individuals with normal, premutation, or fragileX full mutation genotypes. Table 4 below shows the capture immunoassayquantification of FMRP in DBS extracts derived from normal, premutationand Full-mutation individuals.

TABLE 4 mean median minimum maximum Genotype N pM pM Std. dev pM pMRange Females normal 49 25.96 25.40 8.60 11.61 46.29 34.69 Premutation59 22.99 22.49 7.01 9.86 38.73 28.87 Full-mutation 5 17.24 16.74 2.0415.41 20.08 4.67 Total females 113 Males normal 85 25.80 24.79 10.308.61 51.49 42.88 Full-mutation 10 0.55 0.5 0.26 0.2 1.15 0.95 FM mosaic7 3.29 2.90 1.61 1.84 6.58 4.74 Total FM 17 1.70 0.72 1.71 0.20 6.586.39 Total males 102 Total 215

FMRP levels are reported as concentration (pM) in the 50 ul extractswhich are equivalent to 8.7 ul of whole blood. As with lymphocytes,duplicate extracts of 57 DBS were highly correlated (r=0.96), indicatingthat the assay is also very reliable with these extracts. Referring toFIG. 17, on individuals with normal FMR1 alleles, the FMRP level appearsto be normally distributed, with no difference between males andfemales. Moreover, the levels of FMRP followed a normal distribution inthe group composed by both normal and premutation individuals. Referringto FIG. 18 the level of FMRP detected in the assay declines with agefrom infants to pre-teens and then appears to level off in teen yearsand remain unchanged through adulthood.

In males with a full mutation allele, the mean FMRP level was 1.7 pM (6%of normal) with a maximum of 6.6 pM (26% of normal). There was nooverlap between full mutation and normal levels, as seen in FIG. 19A,and the difference between the two groups was highly significant(Mann-Whitney p<0.001). ROC analysis showed that, at a cut off of 7.59pM, sensitivity and specificity were both 100% which indicates that thelikelihood of false negative and false positive results using thisLuminex assay is extremely low. The full mutation male samples includedfull mutation mosaics which have a significantly higher level of FMRP,as seen in FIG. 19B (p=0.001). ROC analysis showed that, at a cut off of1.5 pM, sensitivity and specificity were both 100% which indicates thatthis assay can distinguish mosaic fragile X from non-mosaic. Referringto FIG. 20, the level of FMRP detected in the Luminex assay isconsistent with the intensity of the premutation band in the Southernblot analysis of 2 mosaic full mutation males.

In females with a premutation allele, the mean FMRP level appeared to belower than normal but the difference did not reach significance(p=0.09). The sample population included too few premutation males (2)to distinguish this group from the normal population. Although therewere only 5 full mutation females in the sample population, this groupwas significantly different from normal allele females (Mann-Whitney,p=0.032), and from the combined group of females with either normal orpremutation alleles (Mann-Whitney, p=0.03), as seen in FIG. 21.

The Luminex-based capture immunoassay according to the present inventionreadily identified 14 male Fragile X full-mutation samples among DBSfrom 215 individuals with normal, premutation and full mutation alleles.The identification was accurate and in all cases matched the Fragile Xgenotype determined by Southern blot and PCR. The assay uses a newdeveloped monoclonal antibody, mAb6B8 as capturing antibody, which has ahigh affinity for FMRP and detected no other proteins in western blotsof lymphocytes extracts, as seen in FIG. 14. The new developed rabbitpolyclonal antibody, R477, the detection antibody, also has highaffinity for FMRP. In over-exposed western blots, however, R477 showed avery low cross-reactivity to an unidentified protein (lane 5). Thisprotein is not recognized by mAb 6B8. Since only proteins recognized byboth antibodies are detected in the capture immunoassay, the combinationof mAb6B8 and R477 confers high sensitivity and specificity, and allowsreliable measurements of FMRP in DBS punches derived from blood volumesas low as 2.2 ul (3-mm-diameter punch).

The assay of the present invention allows a low cost, rapid and directquantitative measurement of FMRP that is specific, sensitive, andamenable to high throughput analysis for detection of full mutation FXSand suitable for screening of high-risk population and newborn.

What is claimed is:
 1. An assay for detecting a fragile X mentalretardation protein in a human tissue sample, comprising: a firstantibody having an affinity for a portion of the fragile X mentalretardation protein; and a second antibody having an affinity for adifferent portion of the fragile X mental retardation protein.
 2. Theassay of claim 1, wherein the first antibody comprises a monoclonalmouse antibody.
 3. The assay of claim 2, wherein the first antibody hasan affinity for SEQ. ID NO. 3
 4. The assay of claim 1, wherein thesecond antibody comprises a polyclonal rabbit antibody.
 5. The assay ofclaim 4, wherein the second antibody has an affinity for SEQ. ID NO. 3.6. The assay of claim 1, wherein the first antibody does not have anaffinity for fragile X related protein number one or fragile X relatedprotein number two.
 7. The assay of claim 6, wherein the second antibodydoes not have an affinity for fragile X related protein number one orfragile X related protein number two.
 8. The assay of claim 1, whereinthe first antibody is coupled to a substrate.
 9. The assay of claim 8,wherein the substrate comprises beads.
 10. The assay of claim 8, whereinthe second antibody is coupled to a fluorescent compound.
 11. The assayof claim 10, wherein the fluorescent compound is phycoerythrin.
 12. Amethod of detecting a fragile X mental retardation protein in a humantissue sample, comprising the steps of: providing a first antibody thatis coupled to a substrate, wherein said first antibody is characterizedby an affinity for a first predetermined domain of the fragile X mentalretardation protein; exposing said first antibody to a human tissuesample; introducing a second antibody that is coupled to a fluorescentcompound, wherein said second antibody is characterized by an affinityfor a second predetermined domain of the fragile X mental retardationprotein; detecting whether said first and second antibodies have boundto any fragile X mental retardation protein in the human tissue sample.13. The assay of claim 12, wherein the first predetermined domaincomprises SEQ. ID NO. 3
 14. The assay of claim 12, wherein the secondpredetermined domain comprises SEQ. ID NO.
 1. 15. The assay of claim 12,wherein the first antibody does not have an affinity for fragile Xrelated protein number one or fragile X related protein number two. 16.The assay of claim 15, wherein the second antibody does not have anaffinity for fragile X related protein number one or fragile X relatedprotein number two.